A family of styrylcoumarins: Synthesis, spectroscopic, photophysical and photochemical properties

Article abstract of DOI:10.1002/cplu.201300118

Keeping it in the family: A new family of 5-styrylcoumarins exhibit E-Z isomerization around the C_αi￡C_β bond with large extinction coefficients, medium-lived excited states, and moderate fluorescence quantum yields (see picture). The alteration of these photophysical properties is also corroborated by computational studies. Copyright

Full text of DOI:10.1002/cplu.201300118

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DOI: 10.1002/cplu.201300118
A Family of Styrylcoumarins: Synthesis, Spectroscopic,
Photophysical and Photochemical Properties
Jo¼o Avꢀ,[a] Sꢁrgio Martins,[b] A. Jorge Parola,[a] Jo¼o C. Lima,[a] Paula S. Branco,*[a]
Jo¼o P. Prates Ramalho,[b] and Antꢀnio Pereira*[b]
Photochromism, which is the reversible transformation be-
tween two molecular forms with different absorption spectra,
resulting from photoirradiation, has applicability in several
fields, including light modulation materials, optical recording
materials, optical switches and photochromic inks.[1] The
switching from one molecular state to another is dependent
upon the efficiency of light absorption by the input chromo-
phore and its ability to undergo a photochemical reaction.
Coumarins are an old class of compounds that naturally occur
in several plant families. Owing to their relevant and diverse
pharmacological and biochemical properties,[2] and also owing
to numerous studies related to their application in probes and
fluorescence sensors,[3] interest in this class of compounds is
growing exponentially. Upon substitution with various func-
tional groups at different positions, the coumarin chromo-
phores expand their interesting properties. The usually strong
fluorescence in the visible-light range makes them useful in
laser dyes and organic light-emitting diodes (LEDs).[4] In recent
years, we have devoted some interest to the study of coumarin
chromophores, with particular emphasis on their syntheses
and photophysical properties.[5] A major issue in expanding the
applications of coumarin derivatives is related to their colour
spectrum and intensity of their spectroscopic bands. One solu-
tion arises from increasing the delocalisation of the conjugated
p-electron system; this will allow us to obtain derivatives of
coumarins with absorption bands at longer wavelengths and
with higher molar extinction coefficients. We have been partic-
ularly interested in the extension of the p-delocalised system
of the coumarin chromophore at the 3-position. Our approach
involved 1) building of the 3-vinyl coumarin ring from readily
available substrates[5d] or 2) vinylation of 3-bromocoumarin de-
rivatives.[5a,b] These 3-vinyl coumarins were used as a substrate
for the synthesis of several 3-styrylcoumarins by very simple
and efficient palladium coupling reactions. Depending upon
the substitution pattern, the compounds showed pronounced
bathochromic shifts and hyperchromic effects.[5a,b,d] The palladi-
um-catalysed insertion of 3-bromocoumarin into a number of
alkenes, cycloalkenes and alkynes was also reported by
others.[6] We had the advantage basing our chromophore on
6,7-dihydroxycoumarin (Esculetin), which is a naturally occur-
ring member of the coumarin family.[7] Herein, we propose
studying the effect of the extension of the p-delocalised
system of the 6-methoxycoumarin chromophore at the 5-posi-
tion. The experimental photophysical and -chemical properties
of this new family are correlated with computational studies.
The bromination of 6-methoxycoumarin (1) with Oxone[5b,8]
and HBr, as previously reported,[5b] led to 5-bromo-6-methoxy-
coumarin (2) instead of bromination usually observed at the 3-
position for other coumarin derivatives. Subsequent Suzuki
cross-coupling reaction of 2 with potassium vinyltrifluorobora-
te[5b,9] allowed 6-methoxy-5-vinylcoumarin (3) to be obtained
in very good yield (92%). Although compound 3 was previous-
ly reported by Harayama and Nishita,[10] they did not report
any physical data concerning characterisation of the com-
pound. The extension of the p-electron system to 5-styrylcou-
marins (4) was achieved by Heck palladium coupling reactions
in moderate to high yields (Scheme 1 and Table 1) and, as ex-
pected, the yield decreased if electron-donating groups were
present at the iodoaryl substrate as a consequence of a less fa-
vourable palladium insertion step (4b vs. 4a). As opposed to
the previously reported 3-styrylcoumarins,[5d] the 5-styrylcou-
marin derivatives 4 were obtained as a mixture of both E and
Z isomers (Table 1).
The photophysics of the synthesised coumarins was studied,
and their absorption and emission properties, as well as fluo-
rescence quantum yield (FF), excited-state decay lifetimes (t),
and radiative and non-radiative rate constants (kr and knr, re-
spectively) are summarised in Table 1 (more details are given
in the Supporting Information). The absorption and emission
maxima of the more conjugated 5-styryl derivatives (4a–c) are
all redshifted relative to unsubstituted coumarin 1 or vinyl de-
rivative 3, in accordance with previous studies on 3-styrylcou-
marins.[5d] The extension of the conjugated system in the 5-po-
sition increases both the fluorescence quantum yield and excit-
ed-state lifetime; thus, essentially affecting the long-lived excit-
ed state of these coumarins, behaviour contrary to that exhib-
ited by 3-styryl derivatives, for which more extended
conjugation led to a decrease in both photophysical parame-
ters.[5d] The short fluorescence lifetime component (t1) ampli-
tude (amp) and non-radiative rate constants (knr) are signifi-
cantly larger for coumarins 1 and 3; this suggests that 5-styryl-
coumarins exhibit different, less efficient non-radiative deacti-
vation pathways (Table 1).
[a] J. Avꢀ, Prof. Dr. A. J. Parola, Prof. Dr. J. C. Lima, Prof. Dr. P. S. Branco
REQUIMTE, Departamento de Quimica
Faculdade de CiÞncias e Tecnologia, FCT
Universidade Nova de Lisboa, 2829-516 Caparica (Portugal)
E-mail: paula.branco@fct.unl.pt
[b] S. Martins, Prof. Dr. J. P. Prates Ramalho, Prof. Dr. A. Pereira
Centro de Quꢁmica, Departamento de Quꢁmica
Rua Rom¼o Ramalho, 59, 7000-671 ꢂvora (Portugal)
E-mail: amlp@uevora.pt
The photochemistry of all coumarins was then investigated.
NMR spectroscopy data revealed that, with the exception of 3,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300118.
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Scheme 1. Synthesis of 6-methoxy-5-styrylcoumarins 4. DMF=N,N-dimethylformamide, dppf=1,1’-bis(diphenylphosphino)ferrocene.
Table 1. Synthetic yields, Z/E ratio of stereoisomers, photophysical and photochemical properties of selected coumarins in methanol.
[b]
Compound
R
–
–
Yield[a]
[%]
Z/E ratio[b]
labs
e
lem
FF
tm
t1 [ns] (amp)
t2 [ns] (amp)
kr
knr
Z/E[c] ratio
FP
kP
[nm]
[cmÀ1 mÀ1
]
[nm]
[ns]
[nsÀ1
]
[nsÀ1
]
[nsÀ1
]
275
340
12890
4974
0.2 (0.89)
2.5 (0.11)
1
3
–
–
–
430
462
0.02
0.03
0.3
0.8
0.06
0.04
3.06
1.21
–
–
0.32
–
1.00
–
287
350
13619
3128
0.1 (0.36)
1.1 (0.64)
92
269
308
360
19321
21204
6248
0.8 (0.08)
3.5 (0.92)
4a
H
80
32:68
512
0.12
3.2
0.04
0.27
55:45
0.59
0.18
279
317
370
22619
21689
8363
0.1 (0.08)
4.2 (0.92)
4b
4c
OMe
NO2
50
90
33:67
17:83
576
475
0.30
3.9
0.08
n/a
0.18
n/a
45:55
49:51
0.44
0.75
0.11
n/a
333
22268
<0.01
n/a
n/a
[a] Yield of product isolated. [b] Based on NMR spectroscopy data. [c] Photostationary state, determined from NMR spectroscopy data. n/a=not available.
all coumarins undergo photochemical reactions upon irradia-
tion at appropriate wavelengths. However, there were two dis-
tinct photochemical reactions occurring in these coumarins. Al-
though 1 underwent photodimerisation with a quantum yield
of 0.32 (NMR spectra are given in the Supporting Information),
5-styrylcoumarins 4a, 4b and 4c exhibited E–Z isomerisation
around the exocyclic Ca=Cb bond with relatively high quantum
yields (from 0.44 to 0.75). On the other hand, irradiation of 3
did not give rise to any changes in the NMR or UV/Vis absorp-
tion spectra. This shows that 3 does not undergo photodimeri-
sation and the radiationless deactivation pathway could be
similar to that of the 5-styryl derivatives, which goes through
Figure 1. Frontier orbitals for compounds 3 (top) and (E)-4a (bottom) ob-
rotation around the exocyclic Ca=Cb bond upon irradiation.
This hypothesis is compatible with the observed absence of
changes in the NMR spectra because, in compound 3, there
are no substituents at the b position. Computational studies[11]
show that the exocyclic double-bond length of styrene increas-
es in the first excited state, reducing the double-bond charac-
ter and facilitating rotation. Therefore, a similar non-radiative
deactivation of the excited state can take place in 3. To sup-
port this hypothesis, similar computational studies were con-
ducted for coumarins 3 and 4a. Frontier molecular orbitals for
these compounds are shown in Figure 1. For coumarins 3 and
4, the S1 state is mainly due to the HOMO–LUMO transition,
both for the E and Z isomers. Excitation of the most abundant
tained from density functional theory (DFT) calculations.
E isomer to form the first excited state leads to a change in
the molecular orbital symmetry around CaÀCb. Thus, electron
excitation from the HOMO to the LUMO leads to a reduction
in the electronic density around the C=C bond, and therefore,
to a decrease in the rotational barrier for the excited state, for
both 3 and 4a. These results suggest that the main deactiva-
tion pathway for 3 is similar to that of 5-styrylcoumarins, that
is, rotation around the C=C bond, and thus, the photodimerisa-
tion pathway does not occur in coumarins with vinyl or styryl
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substituents at the 5-position. The fact that the value of knr is
significantly larger for 3, compared with 5-styrylcoumarins,
could arise from the lack of substituents at Cb, which may slow
rotation around the C=C bond.
The relatively high isomerisation quantum yields obtained
for 4a and 4b justify their low fluorescence quantum yields,
compared with their 3-styryl counterparts.[5d] Coumarin 4c has
a very low fluorescence quantum yield, as expected from pre-
vious studies on coumarins with nitro moieties.[5d] From UV
and NMR spectroscopy results, it was determined that samples
from 5-stryrylcoumarins 4 had both E and Z isomers, even
before irradiation (Table 1); this indicates that both isomers are
formed during the final Heck reaction and suggests that the
energy gap between the E and Z species is small. The energy
differences between the E and Z isomers for compounds 4a,
4b and 4c were determined by DFT calculations with different
basis sets and the polarisable continuum model (PCM) for
methanol as the solvent (details are given in the Supporting
Information). In all cases, an energy difference between the E
and Z isomers was found and the E isomer had a lower energy.
The inclusion of zero-point energy (ZPE) corrections slightly re-
duced the energy differences but did not change the trend. If
a larger base (B3LYP/aug-cc-pVDZ) was used, the energy differ-
ence between the isomers was reduced and further inclusion
of solvent slightly increased these energy differences, resulting
in the biggest energy difference between the two isomers for
4b. Nevertheless, the E isomer is the most stable form in all
cases. These results are in agreement with the NMR spectros-
copy data and show that the E isomer is the most abundant
form after synthesis and purification steps for all compounds 4
(Table 1).
Figure 2. Spectral modification of a solution of 4a (2.0ꢃ10À5 m) in MeOH,
upon irradiation at l=360 nm. Inset: variation of the absorbance ratio at
two different wavelengths, l=214 and 308 nm.
whereas electron-withdrawing groups, such as the nitro
moiety, should have the opposite effect.
The isomer ratio at the photostationary state was deter-
mined by NMR spectroscopy to be about 50% for 4a, 4b and
4c (Table 1). Further conversion was not possible, probably
owing to the rather low difference in molar absorptivities of
both isomers at the irradiation wavelengths (De=1143 for 4a
(l=317), 703 for 4b (317), 1502 cmÀ1 mÀ1 for 4c (333 nm)). To
support this hypothesis, time-dependent density functional
theory (TDDFT) calculations for the lowest-lying excited singlet
states were conducted for compound 3 (see details in the Sup-
porting Information). Figure 3 depicts an overlay of the calcu-
lated electronic spectra of both E and Z isomers, as well as
a mixture of the E and Z spectra with the experimentally (NMR
spectroscopy) determined percentages of 0.68 and 0.32, re-
spectively (Table 1). From this curve, maxima at l=266, 300
and 335 nm can be observed that are consistent with the
maxima observed experimentally at l=269, 308 and 360 nm
(Table 1). The observed spectral changes upon irradiation of
compound 4a (Figure 2) can now be justified as a decrease in
To evaluate the applicability of these 5-styrylcoumarin deriv-
atives as photochromic molecules, E–Z isomerisation was also
followed by UV/Vis spectroscopy. Table 1 shows the initial and
photostationary state molar fractions of both isomers, as well
as the photoreaction quantum yields (FP; dimerisation for
1 and isomerisation for 4a–c) and respective rate constants
(kP). Spectral changes observed upon irradiation of a solution
of 4a in methanol are depicted in Figure 2. Because all com-
pounds were isolated as a mixture of isomers, irradiation was
performed at the lowest energy transition to irradiate the most
abundant E isomer preferentially. The decrease in absorbance
at the absorption maximum of l=308 nm, concomitant with
an increase in the absorbance at the lower wavelength part of
the spectrum, is consistent with E–Z photoisomerisation; the Z
isomer is less stable owing to steric hindrance that forces the
aromatic ring out of the plane and prevents more extended
conjugation. The relatively large kP values calculated for 4a
and 4b indicate that photoisomerisation is the main non-radia-
tive pathway of the deactivation of the excited state for 5-styr-
ylcoumarins and justify the low fluorescence of these deriva-
tives. The difference in the isomerisation quantum yields be-
tween coumarins 4a, 4b and 4c can be justified by the nature
of the substituent at the para position of the styryl group. In
principle, electron-donating groups, such as the methoxy
group, should increase the double-bond character of the exo-
cyclic double bond, rendering isomerisation less effective,
Figure 3. Calculated spectra for the Z and E isomers of 4a and the com-
bined spectra of the mixture of isomers with experimental molar fractions.
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the amount of E isomer, which is the species that absorbs pref-
erentially in the l=300 nm region, and an increase in the Z
isomer, which is the isomer that absorbs strongly at higher en-
ergies. The photochemistry of coumarins 4a–c was also tested
in plastic media. After adsorption in a thin N-hydroxyethylacry-
lamide/poly(ethyleneglycol)dimethacrylate copolymer film with
coumarin 4b-adsorbed films, these derivatives were irradiated
and their photoreactivity was followed by spectrofluorimetry.
Visual and spectral modifications of these preliminary results
can be found in the Supporting Information.
a PhD grant (SFRH/BD/65127/2009). The NMR spectrometers are
part of the National NMR Network (RNRMN) and are funded by
the Fundażo para a CiÞncia e a Tecnologia (FCT).
Keywords: coumarins
· density functional calculations ·
isomerization · photochemistry · photochromism
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The pronounced modification of the electronic structure of
the new family of 5-styrylcoumarins in relation to the 3-styryl-
coumarin family resulted in the alteration of the absorption
and emission properties of the chromophore.
Coumarins are usually highly fluorescent molecules that find
applications as dyes and fluorescent probes. Extension of the
conjugated system of these compounds has been used to
yield fluorescent molecules that absorb light at longer wave-
lengths.[5d] However, for the compounds described herein, ex-
tension of the conjugated system at the 5-position yielded
a new type of coumarin derivatives with different photophysi-
cal and -chemical properties. The introduction of 5-styryl moi-
eties rendered coumarins with low to moderate fluorescence
quantum yields. Instead of noteworthy emissive properties,
a new photochemical deactivation pathway through E–Z iso-
merisation around the Ca=Cb bond with reasonable quantum
yields was detected both in liquid and solid media and corro-
borated by computational studies. We anticipate that this be-
haviour could be explored for the use of these compounds in
optical memory devices.
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Acknowledgements
This study has been supported by the Fundażo para a CiÞncia
e a Tecnologia through grant no. PEst-C/EQB/LA0006/2011 and
FCOMP-01-0124-FEDER-007448. J.A. wishes to thank the FCT for
Received: April 3, 2013
Published online on June 20, 2013
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